The aging process is associated with deterioration of the stem cell pool leading to organ failure and dysfunction. Animal models indicate a critical role for DNA repair in the maintenance of the hematopoietic stem cell phenotype. For instance, loss of mismatch repair or double strand break end-joining leads to a competitive repopulation defect and gradual stem cell failure. In mice defective in the ATM gene, and nucleotide excision repair, hematopoietic stem cell dysfunction has also been observed. Likewise, purified hematopoietic stem cells overexpress the mismatch repair protein, MSH-2 and the DNA repair scaffold, XRCC-1. As humans age, the stem cell phenotype changes, with gradual loss of stem cell numbers, regenerative potential and stress response. This proposal will evaluate the hypothesis that loss of genomic stability is an inexorable part of the aging process and is measurable by examining DNA repair pathways and function leads to loss of the stem cell phenotype, loss of regeneration potential and loss of appropriate stress responses. Clinical syndromes that accelerate during aging including anemia, marrow failure yielding mono and pancytopenia, immune dysregulation, myelodysplastic syndromes and leukemias may all have as a common etiology progressive loss of DNA repair capacity in hematopoietic stem cells, as has been observed in animal models. This proposal will develop multiplex assays of a broad range of DNA repair proteins, perform careful assessment of Hematopoietic Stem Cell phenotype and function, and optimize microassays of DNA repair processes in freshly isolated, purified Hematopoietic Stem Cells before and after DNA damage perturbation from the aging population in the decades of 50 to 90. These studies will serve as the prototype with which to analyze normal aging and the diseases associated with aging of hematopoietic stem ceils. Furthermore, these processes undoubtedly affect other tissue stem cells and these approaches may have broad application across stem cell types. These studies will provide the basis to further analysis of DNA repair processes during the aging of stem cells to identify stem cell deficit disorders, response to chemotherapy and other stresses of hematopoietic stem cells land the relation ship between DNA repair processes and stem cell function as measured by conventional and novel assays developed under this RFA.